1. Field of the Invention
The present invention relates generally to the field of mechanical processing of materials such as shape forming of composite materials and more specifically, to an apparatus and method for forming parts by exploiting the anthropomorphic qualities of shape memory alloys.
2. Prior Art
The material that has come to be called shape memory alloys has been known for some time. By way of example, in an article entitled "Shape Memory Alloys" written by L. MacDonald Schetky and appearing in the Scientific American, November 1979 issue beginning at page 74, the author thoroughly explains the mechanical properties of these alloys and indicates that they first came to worldwide attention in 1962. As the author in that article explains, the phenomenon of shape memory alloys is dependent upon the characteristic of certain materials to possess a martensite crystal structure deriving from a parent crystal phase when treated by a certain combination of stress and temperature processing. This martensitic crystal phase can then be transformed back into its parent crystal phase thermoelastically by elevating the temperature of the material. In addition, for the "two-way" shape memory alloys, the phase transformation can be reversed by then lowering the temperature of the material. As a result of these phase transformations the material can be induced to change its shape in a carefully controlled and reversible manner by simply controlling the temperature of the material to induce the aforementioned phase transformations. The article by Schetky illustrates a particular exploitation of this phenomenon for automatically deploying an antenna for a spacecraft. An updated description of the "Shape Memory Effect Alloys" can also be found in the "Encyclopedia of Materials Science and Engineering", volume 6, pages 4365-4374, M.I.T. Press, 1986. However, it is believed that the use of this phenomenon in a press or die for applying reasonably high mechanical pressure forces for pseudo-isostatically pressing one or more parts is a unique application of this phenomenon which has heretofore not been disclosed in the art. The most relevant prior art known to the applicants in this regard will now be disclosed.
U.S. Pat. No. 3,558,369 to Wang et al is directed to a method of effecting a reversion back to an original configuration by means of a martensitic transition of a metal alloy. The alloy is subjected when in an original configuration to deformation at a temperature below a critical temperature to change the shape and then heat the alloy above the critical temperature to effect a reversion.
U.S. Pat. No. 3,579,805 to Kast is directed to a method of forming interference fits by heat treatment. In this patent, members are provided which are formed of a precipitation hardenable alloy composition which undergoes irreversible dimensional changes upon heat treatment. The precipitation hardenable alloy is solution annealed at a temperature higher than the solutioning temperature of the alloy. The members are interfit to form an assembly with a clearance between the members which is less than the irreversible dimensional change and the assembly is subjected to a precipitation hardening heat treatment which causes the irreversible dimensional change.
U.S. Pat. No. 3,832,763 to Schober is directed to a method of drop-forging sintered workpieces. This disclosure provides essentially for a deviation of the shape of the drop forging die wherein a solid cylindrical pre-stressed body is placed into the cavity of a forging die. The cavity of the forging die has a lateral dimension normal to the forging direction wherein this lateral dimension is larger than the diameter of the body. There is no reference to a shape memory alloy die.
U.S. Pat. No. 4,019,925 to Nenno et al is directed to metal articles having a property of repeatedly reversible shape memory effect and the process for preparing such which includes deformation stress to a martensitic alloy.
U.S. Pat. No. 4,036,669 to Brook et al is directed to a mechanical pre-conditioning method and to a mechanical composition capable of undergoing a reversible transformation between the austenitic and martensitic states. The article is deformed from an original configuration into a second predetermined shape from which heat recovery towards the original configuration is desired. Constraining forces are applied to the article and the article is held in the deformed configuration at a predetermined high temperature at which the formation of the martensite in the metallic composition is induced thermally in the absence of applied stress for a predetermined time. This causes a portion of the deformation to be retained as heat recoverable strain.
U.S. Pat. No. 4,149,911 to Clabburn is directed to a memory metal article and a method of making the heat recoverable memory metal member. A stress is applied to the member in a deformed dimensionally heat-unstable state and such temporarily increases the temperature at which formation of austenite begins. This is accomplished while maintaining an applied stress. The article is stored at a temperature less than the higher temperature and the memory metal member then remains in a martensitic state.
U.S. Pat. No. 4,198,081 to Harrison et al is directed to a heat recoverable metallic coupling. A coupling is installed on pipes to be connected and the joint is heated to greater than the transition temperature of the material of the coupling. The coupling is heat recoverable and recovers or shrinks to the heat-stable configuration until it engages the objects and is restrained from further recovery. This creates a tight fit on the object as long as the joint is maintained above the transition temperature. The restraining action of the objects on the coupling introduces non-thermally recoverable plastic deformation stresses into the material of the coupling and when the coupling is cooled to the transition temperature, the stresses are released in spontaneous expansion and the coupling may be removed from the objects.
U.S. Pat. No. 4,283,233 to Goldstein et al is directed to a method of modifying the transition temperature range of a nickel-titanium based shape memory alloys. This is done by the selection of the final annealing temperature. The alloy however is formed into a predetermined permanent shape.
U.S. Pat. No. 4,533,411 to Melton is directed to a method of processing nickel/titanium base shape-memory alloys and structures. The reference teaches cold working an alloy formed of a nickel-titanium based shape memory alloy and provides the alloy in some desired shape while maintaining dislocation-free cells which are obtained in the annealing step. The alloy is deformed in the martensitic state whereby when the alloy is recovered by heating the alloy to the austenitic state and then again cooled to the martensitic state, the alloy will retain the predetermined contour.
Other prior art patents which may be deemed to be relevant to the present invention comprising the following: U.S. Pat. Nos. 3,285,470 Frei et al, 3,622,941 Wetmore, 3,652,969 Willson et al, 3,783,037 Brook et al, 4,035,007 Harrison et al, 4,045,644 Shafer et al, 4,067,752 Brook et al, 4,113,475 Smith, 4,310,354 Fountain et al, 4,365,996 Melton et al, 4,412,872 Albrecht et al, 4,483,174 Goodfellow, 4,489,964 Kipp et al, 4,518,444 Albrecht et al, 4,554,027 Tautzenberger et al.